This invention relates to automatic power control apparatus for amplifying a linearly modulated signal to a constant transmitted power level. The term "linearly modulated" refers herein to modulation of a carrier signal in such a way that the modulated signal has a non-constant envelope. An example of linear modulation is the quadrature phase-shift keying (QPSK) modulation system used in digital communications.
Transmitters for frequency-modulated and other constant-envelope signals employ automatic power control apparatus to maintain a constant transmitted power level despite temperature changes or supply-voltage fluctuations. Prior-art automatic power control apparatus comprises a variable-gain power amplifier, part of the output of which is coupled to a power level detector that produces a voltage signal representing the power level of the amplifier output. The amplifier gain is controlled by feedback of the difference between this voltage signal and a reference voltage to the power amplifier through a loop filter.
When such apparatus is applied to linearly modulated signals, a problem arises. The feedback loop responds to the intrinsic amplitude variations in such signals and attempts to remove these variations by changing the amplifier gain. As a result, signal components that should be transmitted are lost, and the spectrum of the transmitted signal is undesirably broadened.
These problems can be alleviated by lowering the cutoff frequency of the loop filter, but then the transient response of the apparatus suffers. In particular, it takes extra time for the control loop to reach a steady state at the beginning of communication.